Fiber composite materials are produced by a time-consuming and expensive process. The fiber composite material is composed of a proportion of fibers and a proportion of matrix, and the material here is produced during processing. It is by way of example usual, before matrix and fibers are combined, for example by infusion or injection, to produce a dry preform made of fibers and approximating to the final shape of the component. The production of a preform is achieved inter alia by mutually superposing layers of flat semifinished fiber products which, by using pressure and heat, can be pressed into a shape which is similar to the final geometry. The matrix, and with this the prefabricated component, is then hardened.
A matrix material that can be used is thermosets, which can be composed of a plurality of components. Typical examples are epoxy, vinyl, polyester, and phenolic resin systems. These exhibit a curing reaction, which takes place at room temperature or at higher temperatures.
This process has shortcomings in the efficiency of the resources used and the sequence of steps in the process, since individual steps are required in the process for each of the following: preforming, applying release agent to the injection mold, the injection/infiltration process, subsequent downstream operations on the surface, and cleaning of the mold; the process described below links these in order to improve efficiency of resource use.
The production process for fiber composite components gives them a characteristic surface structure which has hitherto restricted their range of applications. The fiber structure here is visible under the surface, because of anisotropy in the direction of the thickness. Production of a smooth surface, required by way of example in the vehicle industry or airline industry, in products intended for everyday use, etc., requires complicated and expensive downstream operations on fiber composite components. By way of example, this requires repeated lacquering of the surface of a fiber composite component, with intermediate curing and smoothing processes. As an alternative, a film can be applied subsequently to a fiber composite component, and to this end by way of example a spray process is used to apply various layers which react chemically and thus form a film. Another possibility for applying films is a thermal or mechanical deep-draw process, where two-dimensional films are heated, and/or are stretched to give the intended geometry, with the reduction of wall thickness.
Because of the enormous weight reduction that can be achieved with fiber composite materials, the automobile industry in particular is also willing to accept expensive measures for providing a high-quality surface, for example integration of intermediate layers, use of comparatively expensive resins with low shrinkage, or use of high-quality semifinished fiber products.
The cost for providing a high-quality surface on fiber composite components here can make up more than 50% of the total costs of the component.
In order to improve the production process, and in order to provide a high-quality surface on a fiber composite component, EP 1 724 098 A1 discloses a process which begins with preforming of a separate layer of material corresponding to the desired fiber shape, applies fiber material to said preformed layer of material, uses a resin to harden the fiber material, and thus forms the final product. The additional layer of material here becomes bonded with the fiber material and in particular forms a desired surface on one side. The preformed additional layer of material can also be transported in the form of preform with the applied fiber material for final processing. The preformed layer of material here serves to some extent as female mold. The component mold itself can be reutilized, and no cleaning is required here to remove resin that has been used. For the additional layer of material, preference is given to the use of plastics films made of PET (polyethylene terephthalate), PC (polycarbonate), PA (polyamide), PMMA (polymethyl methacrylate), PBT (polybutylene terephthalate), PUR (polyurethane), or else acrylic films, or a mixture of the above materials.
DE 103 09 811 A1 discloses a similar process where a mold having the topography of the surface of the finished component is used to produce a plastics film molded in accordance with the final shape of the desired component. A fiber-reinforced plastic is applied to that side of the preformed film which is not the surface of the finished component. The finished component is removed after hardening of the fiber-reinforced plastic.
The same process for producing a fiber composite material provided with a high-quality surface can also be found in DE 10 2008 009 438 A1. The process used for this purpose heats a cut-to-size surface film up to the softening point and molds the same in a mold in accordance with the topography of the molding to be produced. A woven fiber fabric and a polymer resin are applied to the internal side of the preformed surface film, and are molded in accordance with the topography of the molding, and hardened. The finished component is removed from the mold after the hardening process.
A disadvantage of said processes is that separate preforming of the subsequent surface is necessary as an additional operation. Furthermore, the processes provided can provide a high-quality surface only on one side.
In connection with an alternate production process for a fiber composite material, DE 20 2005 005 475 U1 discloses utilization of a profiled preform of sandwich structure. The preform here comprises a stack made of core sublayers and of film sublayers. In the interior there is an unreacted fiber-material-resin layer optionally on a stiffening core. The arrangement moreover has an outer film layer, a nonwoven layer and, for sealing with respect to the environment, a durably elastic film layer. The preform is intended to behave like a film, and it can therefore be shaped by means of a deep-draw system for films, and hardened.
A disadvantage is that the sandwich-structure preform is difficult to transport, because of inadequate mechanical stability. Another possibility moreover is that the resin introduced hardens undesirably during transport.
DE 100 27 129 C1 also discloses a preform for producing a component made of a fiber composite material. The preform here is composed of fiber material which also takes the form of sublayers and which can already have a three-dimensional shape. A sheathing means encloses the fiber material, and is composed of an elastic plastics material. The sheathing means provided has the shape of, for example, a tube or cushion. The previous use of release agent between insert parts and mold can be omitted because the sheathing means has been provided. The sheathing means prevents contact between the mold and the resin. In order to form the finished component, the resin is introduced, in particular by suction, into the interior of the sheathing means, whereupon the fiber material is saturated.
The elastic sheathing means disadvantageously causes undesired prestressing, which can hinder shaping of the component. Although no release agent is required for the mold, a complicated procedure is required to remove the sheathing means from the finished component.